Dunno if this will help, but Vitacost.com has a small alternative-diet section. Their website is old and unweildy, so you need to look carefully through categories of food to find what you need. They are still sold out of a lot of stuff, but are restocking rapidly.
They do have things like potato flour, garbanzo bean flour, gluten-free pie crust mix, protein powders, etc.
They have a 20% off your first purchase option, they do ship to the UK, and the 20% usually takes care of most of the shipping charge (or, it has for us).